This research grant proposes studies on an animal model of obesity which results from eating a high fat diet. This type of obesity is particularly relevant to the high prevalence of obesity which affects the United States and other affluent countries. This grant is designed to provide new insights into the central mechanisms by which one strain of rats (the S5B/PI rat- herein called the RESISTANT strain) avoids obesity when confronted with a high fat diet, whereas a second strain (the Osborne- Mendel rat- herein called the SENSITIVE strain) develops progressive obesity under the same environmental circumstances. When allowed a choice among fat, carbohydrate and protein, the SENSITIVE rats preferred fat and the RESISTANT rats carbohydrate. This finding suggests the hypothesis that the mechanisms in the central nervous system which regulate macronutrient preference differ between the SENSITIVE and RESISTANT rats and that these mechanisms may be related to their sensitivity to develop obesity. We will test the idea that carbohydrate preferring systems are tonically more active in the RESISTANT rat and that fat-preferring systems are tonically more active in the SENSITIVE rat. Anatomic, physiological and molecular approaches will be used to probe the mechanisms underlying the differences in carbohydrate and fat preference in these two strains and identify the basis for their different responses to high fat diets. We have chosen three systems which affect carbohydrate intake and two systems which modulate fat intake. Immunocytochemical analysis of the opioid, the galanin and the NPY system will test the hypothesis that differences between strains may result from anatomic differences related to macronutrient preference. The hypothesis that SENSITIVE and RESISTANT rats have organized their central control of carbohydrate differently will be explored physiologically and pharmacologically by probing the alpha-2 adrenergic system, the serotonergic system and the peptidergic system which uses NPY, since all 3 are known to modulate carbohydrate intake. A similar strategy will be used to dissect the differences in opioid and galanin modulation of fat intake. The simplest integrating hypothesis is that there is a single alteration in one of these systems which accounts for all of the observed effects. We will test this hypothesis by examining the opioid system and its receptors in detail. Alternatively, there may be a single alteration in a regulatory gene that normally coordinates the responses of these systems to the increase in dietary fat. At the molecular level we will use differential hybridization techniques to identify transcriptional genes in the hypothalamus which are suppressed when the RESISTANT rat is fed the high fat diet. This approach is supported by our preliminary finding of a suppressed or absent gene transcript in the hypothalamus of the RESISTANT rat eating the high fat diet. Collectively these experiments should provide critical new insights into anatomic, physiological and molecular mechanisms by which high levels of dietary fat produce obesity.